The creation story - a 6000-year-old earth Or the aftermath of a comet impact?

No, not necessarily. Let's say a comet had struck the earth and created a worldwide flood, not all forms of life would have been wiped out. The animals and humans in high lying areas would have survived. Domesticated animals like sheep and goats would, however, have provided a source of meat, etc., so it would have made sense to take them along.